Anionic Polymerization Mechanism of Acrylonitrile Trimer Anions: Key Branching Point between Cyclization and Chain Propagation

A cluster anion of vinyl compounds in the gaseous phase has served as one of the simplest microscopic models of the initial stages of anionic polymerization. Herein, we describe our investigations into the initial stage mechanisms of anionic polymerization of acrylonitrile (AN; CH₂CHCN) trimer anions. While the cyclic oligomer is found in mass and photoelectron spectroscopic studies of (AN)₃^–, only the chain oligomer is found in the infrared photodissociation (IRPD) spectrum of Ar-tagged (AN)₃^–. On the basis of the calculated polymerization pathway of (AN)₃^–, we consider that the chain oligomers are the reaction intermediates in the cyclization of (AN)₃^–. The rotational isomerization of the (AN)₃^– chain oligomer is found to be the bottleneck in the cyclization of (AN)₃^–. To form the (AN)₄^– chain oligomer by chain propagation, the addition of an AN molecule to (AN)₃^– should occur prior to the rotational isomerization. We conclude that the rotational isomerization in the (AN)₃^– chain oligomer is the key branching point between cyclization (termination) or chain propagation in the anionic polymerization

Ion Imaging of MgI⁺ Photofragment in Ultraviolet Photodissociation of Mass-Selected Mg⁺ICH₃ Complex

We have observed images of MgI⁺ fragment ions produced in ultraviolet laser photodissociation of mass-selected Mg⁺ICH₃ ions at 266 nm. Split distribution almost perpendicular to the polarization direction of the photolysis laser was observed in the photofragment image. Potential energy curves of Mg⁺ICH₃ were obtained by theoretical calculations. Among these curves, the excited complex ion dissociated along almost repulsive potentials with several avoided crossings, which was connected to MgI⁺ + CH₃. In the ground state of Mg⁺ICH₃, the CH₃I was bonded with Mg from the iodine side, and the Mg–I–C bond angle was calculated to be 101.1°. The theoretical results also indicated that the dissociation occurred after the 5²A′ ← 1²A′ photoexcitation, where the transition dipole moment was almost parallel to the Mg–I bond axis. The MgI⁺ and CH₃ fragments dissociated each other parallel to the direction connecting those center-of-masses, which was 67° with respect to the transition dipole moment of 5²A′ ← 1²A′ photoexcitation. Therefore, the fragment recoil direction was assumed to approach perpendicular tendency against the polarization direction under the fast dissociation process. However, calculated potential energy curves showed a complicated reaction pathway for MgI⁺ production, including nonadiabatic processes, although the experimental results indicated the fast dissociation reaction

Structures and CO-Adsorption Reactivities of Nickel Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry

Structures and CO-adsorption reactivities of nickel oxide cluster cations were investigated by ion mobility mass spectrometry. The series of Ni_<i>n</i>O_<i>n</i>–2⁺, Ni_<i>n</i>O_<i>n</i>–1⁺ and Ni_<i>n</i>O_<i>n</i>⁺ cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into an ion-drift cell. From the arrival time distributions of Ni_<i>n</i>O_<i>n</i>⁺ and Ni_<i>n</i>O_<i>n</i>–1⁺ in the ion mobility spectrometry, structural transition from two-dimensional (2D) ring to three-dimensional (3D) compact structures was found at <i>n</i> = 5. In addition, 2D and 3D structural isomers were found to coexist for Ni₅O₅⁺, Ni₆O₅⁺ and Ni₇O₆⁺. By adding CO gas to buffer gas in the ion-drift cell, Ni₄O₃⁺ and Ni₅O₄⁺ cluster cations were found to be more reactive for the CO adsorption reactions than Ni₄O₄⁺ and Ni₅O₅⁺. Under the pseudo-first-order approximation, rate constants for CO-adsorption were determined to be (8.4 ± 0.7) × 10^–11 cm³ molecule^–1 s^–1 for Ni₄O₃⁺ and (9.6 ± 0.8) × 10^–11 cm³ molecule^–1 s^–1 for Ni₅O₄⁺. These rate constants are 2 orders of magnitude faster than those for Ni₄O₄⁺ and Ni₅O₅⁺, which have reported previously. These differences of rate constants can be originated in the structures of the nickel oxide cluster ions

Compositions and Structures of Vanadium Oxide Cluster Ions V_<i>m</i>O_<i>n</i>^± (<i>m</i> = 2–20) Investigated by Ion Mobility Mass Spectrometry

Stable compositions and geometrical structures of vanadium oxide cluster ions, V_<i>m</i>O_<i>n</i>^<i>±</i>, were investigated by ion mobility mass spectrometry (IM-MS). The most stable compositions of vanadium oxide cluster cations were (V₂O₄)­(V₂O₅)_{(<i>m</i>−2)/2}⁺ and (VO₂)­(V₂O₅)_{(<i>m</i>−1)/2}⁺, depending on the clusters with even and odd numbers of vanadium atoms. Compositions one-oxygen richer than the cations, such as (V₂O₅)_<i>m</i>/2^– and (VO₃)­(V₂O₅)_{(<i>m–</i>1)/2}^<i>–</i>, were predominantly observed for cluster anions. Assignments of these stable cluster ion compositions, which were determined as a result of collision-induced dissociations in IM-MS, can partly be explained with consideration of spin density distribution. By comparing the experimental collision cross sections (CCSs) obtained from ion mobility measurement with CCSs of the theoretically calculated structures, we confirmed the patterned growth of geometrical structures partially discussed in previous theoretical and spectroscopic studies. We showed that even sized (V₂O₅)_<i>m</i>/2^<i>±</i> where <i>m</i> = 6–12 had right polygonal prism structures except for the anionic V₁₂O₃₀^<i>–</i>, and for the clusters of odd numbers of vanadium <i>m</i>, cations and anions can either have bridged or pyramid structures. Both of the odd sized structures proposed were derivatives from the even sized right polygonal prism structures. The exception, V₁₂O₃₀^<i>–</i>, which had a CCS almost equal to that of the neighboring smaller V₁₁O₂₈^<i>–</i>, should have a structure of higher density than the right hexagonal prism, in which it was proposed to be a captured pyramid structure, derived from V₁₁O₂₈^<i>–</i>

Small Carbon Nano-Onions: An Ion Mobility Mass Spectrometric Study

Structures and charges of nanocarbon cluster ions, C_<i>n</i>^<i>z</i>+ (100 ≤ <i>n</i> ≤ 800, <i>z</i> = 1 and 2), have been determined using ion mobility mass spectrometry. For singly charged ions, a compact cluster ion series was observed in addition to monolayer fullerene ions for <i>n</i> = 260–700 continuously. Previous electron microscopic observations indicated that the compact clusters were bilayer fullerenes (nano-onions), in which the inner and outer layers grow from a structure close to [C₃₀@C₂₃₀]⁺ at <i>n</i> = 260. The present study also suggests that several combinations of inner and outer layer fullerenes were produced. The results indicated that the interlayer distance depended on different combinations of inner and outer layers and that the observed lower limit of the interlayer distance agreed well with that of graphite (3.35 Å). The upper limit corresponded to bilayer structures in which the number of atoms of the inner layer was constant at about 30, the smallest fullerene size observed in this study. Series of monolayers and compact bilayers of doubly charged ions with cross sections that coincided with those of monocations were observed in nearly the same size region as monocations

Correlation between Electronic Shell Structure and Inertness of Cu_<i>n</i>⁺ toward O₂ Adsorption at <i>n</i> = 15, 21, 41, and 49

The inertness of metal clusters in air is important for their application to novel materials and catalysts. The adsorption reactivity of copper clusters with O₂ has been discussed in connection with the electronic structure of clusters because of its importance in electron transfer from the cluster to O₂. Mass spectrometry was used to observe the reaction of Cu_<i>n</i>⁺ + O₂ (<i>n</i> = 13–60) in the gas phase. For O₂ adsorption on Cu_<i>n</i>⁺, the relative rate constants of the <i>n</i> = 15, 21, 41, and 49 clusters were clearly lower than those with other <i>n</i>. Theoretical calculations indicated that the inertness of Cu₁₅⁺ with 14 valence electrons was related to the large HOMO–LUMO gap predicted for the oblate Cu₁₅⁺ structure. The Clemenger–Nilsson model was used to predict that the electronic subshell of oblate Cu₄₉⁺ with 48 electrons was closed. This electronic shell closing of Cu₄₉⁺ corresponds to the inertness for O₂ adsorption